LED printers and laser printers using light emitting diode (LED) heads, semi-conductor laser heads, etc., to generate light signals which are irradiated onto the surface of light-sensitive materials to form static latent images from which printed graphic images are formed on printing materials are already well known. LED printers, particularly, have recently been the object of widespread attention because they offer advantages such as the possibility of smaller device designs and lower manufacturing costs than laser printers.
Such LED printers, as shown in FIG. 21, consist of: a charging device 102 which electrifies the surface of a revolving light-sensitive drum 100; an LED print head 104 which irradiates light to form static latent images in accordance with the input signals, i.e., the electric signals on the charged light-sensitive sensitive drum 100; a processing device 106 to process the static latent Images; a transcriber 110 to transfer toner, the image formation medium, onto the printing material 108, which is moved in accordance with the revolution of the light sensitive drum 100; a fixing device (not shown in drawing) to fix the toner that is transferred onto the printing material 108 by heating, etc.; and a cleaner 112 to clean the surface of the light-sensitive drum 100.
An LED print head 104 used in such LED printers consists of a substrate 114 on which is formed an electrical circuit, an LED array 116 consisting of LED's which are formed on the substrate and which generate light in accordance with the applied electric signals, and a rod-lens array 118 consisting of cylindrical lenses which condense light generated from the LED array 116 onto the light-sensitive drum 100. These components are assembled in such a way that light generated from the LED array 116 is condensed onto the surface of the electrified light-sensitive drum 100 by means of the rod-lens array 118, such that latent images for graphic images to be formed on the printing material 108 are formed on the light-sensitive drum 100.
The LED array 116 on the LED print head 104, as shown in FIG. 22, consists of LED array chips 16a, 16b, etc., set apart by a certain distance 1, each chip having a certain number of LED's arranged at a certain pitch P. The LED array chips 16a, 16b, etc., consist of a chip substrate 122 and a number of light-emitting elements, LED's 124, which are formed on the surface of the chip substrate 122. An electrode 126 made of a conducting metal is connected to the surface of each LED 124. At the other end of the electrode 126 is formed a pad electrode 128, which is electrically connected via a wire (not shown in drawing) to a driver IC (not shown in drawing) mounted on the substrate 114.
Along with recent developments in office automation products, the demand for LED print heads with improved performance, particularly with regard to resolution, has also increased. Resolution is determined by the density of pixels (dpi) forming the image printed on the printing material, and is thus a function of the density of the LED's used as light-emitting elements. However, attaining resolutions above a certain level, such as 480 dpi, using LED print heads is extremely difficult in practice due to limitations in manufacturing precision, as is explained below.
In other words, in order to obtain a resolution or pixel density of 600 dpi, for instance, a pitch P of approximately 42 .mu.um for the LED's 124 on LED array chips 16a, 16b, etc. , is necessary, 20 .mu.m being the columnwise width W of an LED 124 of conventional size. Assuming that the distance d from the columnwise edge of the chip to that of the neighboring LED 124 is 8 .mu.m, practically the limit of manufacturing precision, the clearance between LED array chips 16a, 16b, etc. would be approximately 6.mu.m.
Furthermore, the cutting of the stick-shaped chip substrate on which LED 124 is formed into individual LED array chips would generally call for a cutting precision in the order of .+-.5 .mu.m. Moreover, a margin of error of at least .+-.10 .mu.m when die bonding the cut individual LED array chips 16a, 16b, etc., onto the circuit substrate must also be considered. In view of the above limitations in manufacturing precision, it can be seen that realizing a resolution of 600 dpi would be extremely difficult if not altogether impossible.
Thus, in attempting to manufacture LED print heads 104 with above a certain level of resolution, since dimensional errors in cutting a chip substrate stick into LED array chips 16a, 16b, etc. and errors in die bonding onto circuit substrates inevitably occur during the manufacturing process, and particularly since it is extremely difficult to set the LED's 124 on the end of neighboring LED array chips 16a, 16b, etc. such that the distance or pitch between them is identical to that between the other LED's 124, that is, since it is extremely difficult to manufacture LED array chips 16a, 16b, etc. such that all the LED's 124 are formed at a fixed pitch at high density, it has not been possible to satisfy the demand for affordable high resolution LED print heads.
Therefore, the object of the invention is to provide an LED print head with improved resolution using relatively simple methods.